At E3, Microsoft announced a South Park game for XBLA - without mentioning the developer. Here's a (somewhat lame) teaser trailer. Could this be the "mystery franchise" we've been speculating about?
With no gameplay images and no additional information it's hard to tell, but it would certainly fit like a glove. It's as huge as implied, it would explain the secrecy and the license would fit Telltale well. Also, XBLA suits episodic (no confirmation yet on this one being episodic, though) and we know Telltale doesn't like to be bound to any single distribution platform.
It's still speculation, but it's easily the most plausible theory so far.